1. Field of the Invention
This invention is concerned with an improved process for converting synthesis gas, i.e., mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures. In one aspect, this invention is particularly concerned with a process for converting synthesis gas substantially directly to hydrocarbon mixtures rich in aromatics.
2. Prior Art
Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966), Interscience Publishers, New York, New York, the contents of which are incorporated herein by reference. The technique for gasification of coal or other solid, liquid or gaseous fuel are not considered to be a part of this invention.
It is well known that synthesis gas comprising carbon monoxide and hydrogen will undergo conversion to form reduction products of carbon monoxide, at temperatures in the range of 300.degree. F. to about 850.degree. F. and pressures in the range of one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been extensively studied, produces a range of hydrocarbons, waxy materials and some liquid materials which have been used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides of iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium.
The range of catalysts and catalyst modifications disclosed in the art encompass an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen and provide considerable flexibility toward obtaining selected boiling-range products. Nonetheless, in spite of this flexibility, it has not been possible heretofore to provide a catalyst for medium pressure operation (5-30 atm) which will produce particularly olefin compositions comprising primarily beta double bond characteristics and boiling in the gasoline boiling range. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions," Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4, pp. 446-488, Interscience Publishers, New York, New York.
It has been discovered that synthesis gas may be converted to oxygenated organic compounds and these compounds then converted to higher hydrocarbons, particularly high octane gasoline, by contacting synthesis gas with a carbon monoxide reduction catalyst followed by contacting the conversion products so produced with a special type of zeolite catalyst in a separate reaction zone. This two-stage conversion is described in copending U.S. patent application Ser. No. 387,220, filed Aug. 9, 1973. Compositions of iron, cobalt or nickel deposited in the inner absorption regions of crystalline zeolites are described in U.S. Pat. No. 3,013,990. Attempts to convert synthesis gas over X-zeolite base exchanged with iron, cobalt and nickel are described in Erdol and Kohle-Erdgas, Petrochemie; Brennstoff-Chemie, Vol. 25, No. 4, pp. 187-188, April 1972.
In copending application Ser. No. 733,982, filed Oct. 20, 1976, there is disclosed a process for the conversion of syngas to various hydrocarbon products utilizing an intimate mixture of a Fischer-Tropsch component and a ZSM-5 type zeolite. The process of said copending application is a one-stage process and among the hydrocarbons that can be produced are those which are rich in aromatics.
Although the process of this copending application is indeed effective in producing products having a substantial quantity of aromatics, nevertheless, there are disadvantages associated with said process, primarily in the regeneration aspect of the catalyst. It is known that when processes of this type are operated under conditions which favor the production of aromatics that there is also produced substantial amounts of coke which are deposited about the acid ZSM-5 catalyst. This requires that the catalyst be subjected to frequent regeneration, and due to the fact that the process of said copending application Ser. No. 733,982 involved a catalyst mixture containing a Fischer-Tropsch component such as an iron catalyst and a ZSM-5 catalyst, the extent and amount of regeneration was limited by the effect that the regeneration would have on the iron component. Thus, although HZSM-5 by itself exhibits a remarkable stability with regard to regeneration of the same by burning off carbon deposits, the same is not true with respect to a Fischer-Tropsch catalyst, in general, and iron and cobalt containing catalysts in particular.
It is also to be understood that another difficulty in the area of regeneration stems from the fact that those conditions which are optimum for the regeneration of an acidic catalyst such as ZSM-5 are not necessarily optimum for the regeneration of a Fischer-Tropsch component. Other difficulties which are experienced in a one-stage process involving the production of aromatics are due to the nature of the chemistry which is involved. Thus, for example, in order to produce aromatics a ZSM-5 type zeolite has to "work harder" than it would if an olefinic product were desired. Thus, a greater degree of severity with regard to reaction conditions is required for the production of aromatics than would be required for the production of olefins. As has heretofore been stated, the production of aromatics also deposits more coke on the catalyst such that it deactivates faster than would be the case if an olefinic product were produced. Thus, it can be seen in those situations where an intimate mixture of an iron or cobalt containing Fischer-Tropsch component and a ZSM-5 zeolite were used to convert syngas, the catalyst would have to be regenerated more frequently when aromatic hydrocarbons were produced. This in turn would further complicate the situation since as has heretofore been stated the conditions for the regeneration of the Fischer-Tropsch component are not necessarily the conditions required for the regeneration of the zeolite component.
Additionally, while not wishing to be bound by any theory of operation, it nevertheless appears that the cleaner the feed material is to a ZSM-5 type zeolite, the longer that it will last in converting said feed to aromatics.